1. Field of the Invention
The present invention concerns a novel method for preparing polymers (preferably (meth)acrylate polymers and copolymers) having a narrow molecular weight distribution by metal-free anionic polymerization in the presence of an organic cation containing at least one amino substituent and one aromatic substituent on a phosphorus atom.
2. Discussion of the Background
In the last decade, great interest has emerged concerning the polymerization of (meth)acrylates (i.e. methacrylates or acrylates), particularly methyl methacrylate (MMA), under ambient conditions. A number of new polymerization systems and processes have been reported, and a recent review covers most of the literature on the polymerization of MMA at ambient temperatures (T. P. Davies, D. M. T. Haddelton, S. N. Richards, J. M. S.-Rev. Macromol. Chem. Phys., 1994, C34, 243).
The polymerization of acrylic acid derivatives at temperatures of more than 20.degree. C. and particulary between 40 and 120.degree. C. in the presence of an initiator corresponding to or containing the structure I ##STR2## where E is CR.sup.IV R.sup.V, wherein the C together with R.sup.IV and R.sup.V forms an unsaturated carbocyclic ring was disclosed in DE-A 36 32 361 and DE-A 37 00 195. The initiator (I) can also be used in its protonated form. As shown in the examples, the best results are obtained when the initiator is employed as a Lewis acid adduct. Merely very poor yields, incomplete monomer conversion and polymers having a broad molecular weight distribution are obtained when the reaction is performed in the presence of the neutral catalyst. In the case of the protonated species the acidic proton may cause chain termination. Therefore, the versatility of the polymerization process using (I) is limited.
Another process for metal-free anionic polymerization of acrylates and methacrylates disclosed by Seebach and Pietzonka (Angew. Chem. Int. Ed. Engl. 1993, 32, 716) makes use of the P4-phosphazene base which forms a very soft and bulky cation. The initiating system can for example be prepared by treating ethyl acetate with the P4-phosphazene base. Polymerization of for example, methylmethacrylate is accomplished at 60.degree. C. furnishing predominantly a syndiotactic polymer material. However, the P4-phosphazene base is not readily available and therefore not favorable for large scale production. In addition, the formation of block copolymers by this process has not been disclosed.
A distinct improvement over the methods described is achieved by the anionic polymerization in the presence of phosphonium counterions as for example Ph.sub.4 P.sup.+ Ph.sub.3 C.sup.-, described in WO 96/28487 (U.S. patent application Ser. No. 08/609,732). According to WO 96/28487 the polymerization is carried out at temperatures up to 40.degree. C. and leads to (meth)acrylate polymers and copolymers having a narrow molecular weight distribution in good yields. One disadvantage of this process is the sensitivity of the initiator, which decomposes within a few minutes at room temperature.
U.S. patent application Ser. No. 08/827,906, filed, which is included herein by reference, describes a method for preparing polymers by anionic polymerization in the presence of phosphonium cations containing hydrocarbon substituents, wherein at least one is an annelated aromatic ring, as substituents at the phosphorus atom. The polymerization is carried out at temperatures above 40.degree. C., because of the high stability of the initiator.
Thus, a need exists for a method for producing (meth)acrylate polymers and copolymers having a narrow molecular weight distribution (so-called "monodisperse" polymers), which provides effective control of polymer molecular weight, of molecular weight distribution and of polymer stereoregularity, in good yields at ambient temperatures.